1. Field of the Invention
The present invention relates to filter traps for Diesel particulate matter, and more particularly to an improved system and method therefor for removing Diesel particular matter (DPM) and unburned hydrocarbons (UHC) from the exhaust of Diesel engines. Still more particularly, the present invention relates to a Diesel exhaust filtration system and method which utilizes dual DPM/UHC traps, low temperature regeneration ignition of the DPM and associated UHC to burn the DPM and UHC into CO and CO.sub.2 gases and recirculation of the burned DPM and UHC back into the engine intake for subsequent burning of any remaining UHC in the engine.
2. Description of the Prior Art
Diesel engines provide an efficiency advantage over conventional gasoline engines, and for this reason Diesel engines are a preferred power-plant for many applications. Indeed, Diesel engines are becoming ever more likely to be chosen for automotive use because of their higher efficiency. However, it is a well recognized problems that Diesel engines produce a soot, known as Diesel particulate matter (DPM). DPM is offensive to the average person, especially the person who is first behind a bus or other large engined vehicle. But, actually, DPM is composed mainly of carbon which, though not harmless, is less hazardous than unseen unburned hydrocarbons (UHC) which are known to be carcinogens. Thus, for both the sake of the environment and the health of the public it is very desirable to reduce or eliminate both DPM and UHC from the exhaust of Diesel engines.
In the various devices proposed in the prior art, there is provided a DPM trap consisting in one form or another of a perforated metallic tube covered by a ceramic fiber filter. The metallic tube provides mechanical strength, while the ceramic fiber filter performs the actual DPM removal. As engine operation proceeds, the DPM trap accumulates progressively more DPM. Problematically, the DPM eventually clogs the DPM trap resulting in loss of engine performance and, if left to continue unabated, would cause the engine to stall. Accordingly, there is provided some means for periodically removing the accumulated DPM from the DPM trap. This process of DPM removal is referred to as "regeneration". Regeneration is typically accomplished by burning the DPM at or above its ignition temperature of around 600 degrees Centigrade (in the presence of oxygen), which converts the DPM into CO and CO.sub.2. During regeneration, the Diesel exhaust may dump to the atmosphere, or a second DPM trap may be utilized in a cyclic fashion of operation. It will be appreciated in view of the foregoing, that a ceramic fiber filter is preferred as it can withstand the temperatures associated with regeneration, yet can trap DPM which typically are on the order of 0.1 to 0.3 micrometers in size.
Two different approaches have been taken to accomplish periodic regeneration of DPM traps. The first is to operate the DPM Trap as close as possible to the DPM ignition temperature, the second is to operate the DPM trap at lower than the DPM ignition temperature.
The theory behind operating a DPM trap near the ignition temperature of DPM is to permit rapid regeneration with very little additional energy being needed to provide ignition. The operating temperature of the DPM trap is maintained high by locating it near the exhaust manifold, and either fuel or electricity is introduced to initiate ignition of the DPM. In some systems, the Diesel engine RPM is increased so as to provide a suitable hot exhaust gas for ignition of the DPM. Operation of the DPM trap near the DPM ignition temperature, while providing suitable burning of the DPM, results in a very rapid burn process. This frequently leads to DPM trap failure due to thermal shock, shortened life, melt-down, or poor operation under certain driving modes.
The theory behind operating a DPM trap below the ignition temperature of the DPM is to provide regeneration which is less injurious to the DPM trap. In order to achieve ignition of the DPM at a temperature below 600 degrees Centigrade, a DPM oxidation promoting catalyst is introduced into the fuel, added as an exhaust gas chemical agent upstream of the DPM filter, or as a pre-treat for the ceramic fiber filter of the DPM trap. These catalysts are certain metallic compounds, most notably composed of lead, copper, manganese, or noble metals, such as platinum and palladium.
Specific examples of the prior art will now be given.
U.S. Pat. No. 4,576,617 to Renevot, dated Mar. 18, 1986, discloses a Diesel exhaust DPM trap which utilizes a regeneration process in which a very flammable mixture, such as methyl alcohol, is introduced into the DPM trap which, in combination which a glow plug, affects ignition of the DPM. Different mixtures may be used depending on whether the filter of the DPM trap is impregnated with a catalyst, such as either platinum or palladium, as the ignition temperature of the DPM will be different in accordance therewith.
U.S. Pat. No. 4,631,076 to Kurihara et al, dated Dec. 23, 1986, discloses a DPM trap which utilizes regeneration based upon ignition of the DPM caused by selective introduction of catalytic solutions into the exhaust gas upstream of the DPM trap. Examples of suitable metal catalytic compounds include Pd(NH.sub.4).sub.3 (OH).sub.2 and Cu(NH.sub.3).sub.4 (OH).sub.2.
U.S. Pat. No. 4,685,291 to Ha, dated Aug. 11, 1987, and U.S. Pat. No. 4,813,233 to Vergeer et al, dated Mar. 21, 1989, disclose a dual DPM trap system in which periodic regeneration may be achieved at lower than 600 degrees centigrade, where a by-pass conduit allows selectively for heated or cooled exhaust gasses to enter the DPM traps. Four different ways to achieve regeneration are disclosed, as follows. 1) Each of the DPM traps are located remote from the engine, but each is selectively heated by the other's exhaust manifold. When not heated, UHC can accumulate on the pores of the trapped DPM. To effect regeneration, heat from the other DPM trap's exhaust manifold is used to induce ignition of the DPM, where it is believed by the inventor that the UHC serves as a fuel to assist ignition of the DPM at temperatures as low as 250 degrees Centigrade. Exhaust coolers may be used in place of remote placement of the DPM traps. 2) For two-stroke Diesel engines, regeneration is induced by a synergism between the scavenging blower system and introduction of finely atomized fuel above the DPM traps, with a diesel fuel additive being used, such as manganese in concentrations on the order of 100 mg/L of diesel fuel. 3) Regeneration is induced by introduction of finely atomized fuel combined with air above the DPM traps, with a diesel fuel additive being used, such as manganese in concentrations on the order of 80 to 100 mg/L of diesel fuel, or copper. 4) Again, for two-stroke Diesel engines, regeneration is induced by a scavenging blower system which controls the scavenging ratio of the engine, and introduction of finely atomized fuel above the DPM traps, regeneration occurring because of increased exhaust gas temperature at medium load speed conditions, a diesel fuel additive being used, such as manganese in concentration is on the order of 100 mg/L of diesel fuel.
U.S. Pat. No. 4,720,972 to Rao et al, dated Jan. 26, 1988, discloses a dual DPM trap utilizing a heat exchanger to cool the exhaust gases to the range between 200 and 500 degrees Fahrenheit, which produces condensation of UHC upon the DPM at the DPM trap. The DPM trap uses a catalytically coated ceramic fiber or wire mesh, where the catalytic material may comprise SO.sub.2 active oxidation catalyst such as platinum, tungsten or paladium-platinum coated on a porous, cellular cordierite body. Electrical heating is used to initiate ignition, and burn front will progressively move down the DPM trap from the ignition location until regeneration concludes in 6 to 9 minutes.
U.S. Pat. No. 4,730,454 to Pischinger et al, dated Mar. 15, 1988, discloses a DPM trap in which regeneration is effected by regulating the DPM concentration which lies within the explosive range of the DPM/exhaust mixture by briefly adding or recycling combustible particulates to the exhaust gas flow at the DPM trap. A secondary source of energy, such as electrical, is used to supply ignition, from which an explosive wave runs progressively through the DPM trap.
While the schemes for cleaning Diesel exhaust are effective to remove DPM, there is presently no successful system which can effectively remove both DPM and UHC from Diesel exhaust.